Storing electricity as electricity is currently possible only in capacitors. A lot of fuss is made about the potential of super-capacitors and ultra-capacitors to scale up, but little has happened yet at the grid level.

Almost everything else involves converting electricity to a different kind of energy; and then converting it back to electricity when needed. There's an efficiency hit with each conversion.

I'll set out the different types of storage, classified by the form of converted energy that is stored.

At the grid level as of 2012 (time of writing), pumped hydro storage (storage - minutes to weeks) and virtual storage (storage - seconds to hours) are the main forms of storage on electricity grids, with a tiny tiny proportion of chemical storage (storage minutes to hours). Everything else is at prototype stage, but may well have gone mainstream by the time you read this, future reader. Flywheels would provide storage at the scale of seconds; pressure and thermal storage at the scale of hours to days;

Gravitational potential

There's only one form of electrical storage done at significant grid scale globally, and that's pumped hydroelectric storage.

Loss rate: There's very little time-based losses. The round trip (electricity now to electricity later) is about 75-80% efficient.

Chemical potential

Batteries are the next biggest form of storage, globally, but because they're more expensive than pumped hydro storage, there's not much grid-connected stuff - they're more used for portable applications. Lithium-ion were the hottest kids on the block, but there's a lot of innovation out there, with vanadium flow batteries, Lithium-iron, and the work of Jay Whitacre and others.

Loss rate: Time-based losses vary by technology, and can be 10% per day, or 1% per week. The round trip (electricity now to electricity later) is about 70-80% efficient.

Thermal storage

There are a three variants on this. All, as far as I know, are at prototype stage only. Each is effectively a reversible carnot engine, though each performs well below ideal carnot efficiency on both stages.

heat storage, e.g. as molten salts, as proposed for some concentrating solar plants.

combined heat & cold storage, such as used by Isentropic, where two separate blocks of mass (e.g. gravel) are used, one for heat, one for cold.

cold storage, such as the Highview system, which liquifies and stores atmospheric nitrogen, and uses its expansion upon heating to ambient temperature, to drive a turbine.

Loss rate: It's too early to tell what the time-based losses are for each method, but it will probably be significant at the scale of days to weeks. The round trip (electricity now to electricity later) is about 25% efficient currently, but with scope for improvement, particularly if waste heat is available nearby.

Pressure storage

CAES - Compressed Air Energy Storage, has been proposed, and prototyped in various forms: compressed air in underground caverns, or in subsea inflatable balloons.

Loss rate: There will be time-based losses. There's an almost-immediate loss, as the heat generated during compression dissipates. And the round trip efficiency is likely to be low - under 50%.

Virtual storage

Demand-side response is billed as the biggest storage resource waiting to be harnessed, and has been implemented already in quite a few places. There are a few good reasons for that billing. It works just like energy storage, as far as the grid is concerned: if I heat my hot-water tank in 6 hours time, rather than right now, I'll use 10kWh of energy in 6 hours, rather than now: and that means that the grid has 6kWh more energy now for other uses, and 6kWh of energy less, in 6 hours time. So that's virtual storage effectively at 100% efficiency. This is the "demand-side response" part of the "smart grid" you might have heard about: and electricity grids have beeen using various forms of it since the 1970s. It's hugely scalable, for two reasons:

water and space heating demand is much much higher than total non-heat electricity demand; and